package envoy

import (
	"errors"
	"flag"
	"fmt"
	"net"
	"os"
	"os/exec"
	"strings"

	"github.com/mitchellh/cli"
	"github.com/mitchellh/mapstructure"

	"github.com/hashicorp/consul/agent/structs"
	"github.com/hashicorp/consul/agent/xds"
	"github.com/hashicorp/consul/agent/xds/proxysupport"
	"github.com/hashicorp/consul/api"
	proxyCmd "github.com/hashicorp/consul/command/connect/proxy"
	"github.com/hashicorp/consul/command/flags"
	"github.com/hashicorp/consul/ipaddr"
	"github.com/hashicorp/consul/tlsutil"
)

func New(ui cli.Ui) *cmd {
	c := &cmd{UI: ui}
	c.init()
	return c
}

const DefaultAdminAccessLogPath = "/dev/null"

type cmd struct {
	UI     cli.Ui
	flags  *flag.FlagSet
	http   *flags.HTTPFlags
	help   string
	client *api.Client

	// flags
	meshGateway           bool
	gateway               string
	proxyID               string
	nodeName              string
	sidecarFor            string
	adminAccessLogPath    string
	adminBind             string
	envoyBin              string
	bootstrap             bool
	disableCentralConfig  bool
	grpcAddr              string
	envoyVersion          string
	prometheusBackendPort string
	prometheusScrapePath  string

	// mesh gateway registration information
	register           bool
	lanAddress         ServiceAddressValue
	wanAddress         ServiceAddressValue
	deregAfterCritical string
	bindAddresses      ServiceAddressMapValue
	exposeServers      bool
	omitDeprecatedTags bool

	gatewaySvcName string
	gatewayKind    api.ServiceKind
}

const meshGatewayVal = "mesh"

var defaultEnvoyVersion = proxysupport.EnvoyVersions[0]

var supportedGateways = map[string]api.ServiceKind{
	"mesh":        api.ServiceKindMeshGateway,
	"terminating": api.ServiceKindTerminatingGateway,
	"ingress":     api.ServiceKindIngressGateway,
}

func (c *cmd) init() {
	c.flags = flag.NewFlagSet("", flag.ContinueOnError)

	c.flags.StringVar(&c.proxyID, "proxy-id", os.Getenv("CONNECT_PROXY_ID"),
		"The proxy's ID on the local agent.")

	c.flags.StringVar(&c.nodeName, "node-name", "",
		"[Experimental] The node name where the proxy service is registered. It requires proxy-id to be specified. ")

	// Deprecated in favor of `gateway`
	c.flags.BoolVar(&c.meshGateway, "mesh-gateway", false,
		"Configure Envoy as a Mesh Gateway.")

	c.flags.StringVar(&c.gateway, "gateway", "",
		"The type of gateway to register. One of: terminating, ingress, or mesh")

	c.flags.StringVar(&c.sidecarFor, "sidecar-for", os.Getenv("CONNECT_SIDECAR_FOR"),
		"The ID of a service instance on the local agent that this proxy should "+
			"become a sidecar for. It requires that the proxy service is registered "+
			"with the agent as a connect-proxy with Proxy.DestinationServiceID set "+
			"to this value. If more than one such proxy is registered it will fail.")

	c.flags.StringVar(&c.envoyBin, "envoy-binary", "",
		"The full path to the envoy binary to run. By default will just search "+
			"$PATH. Ignored if -bootstrap is used.")

	c.flags.StringVar(&c.adminAccessLogPath, "admin-access-log-path", DefaultAdminAccessLogPath,
		fmt.Sprintf("The path to write the access log for the administration server. If no access "+
			"log is desired specify %q. By default it will use %q.",
			DefaultAdminAccessLogPath, DefaultAdminAccessLogPath))

	c.flags.StringVar(&c.adminBind, "admin-bind", "localhost:19000",
		"The address:port to start envoy's admin server on. Envoy requires this "+
			"but care must be taken to ensure it's not exposed to an untrusted network "+
			"as it has full control over the secrets and config of the proxy.")

	c.flags.BoolVar(&c.bootstrap, "bootstrap", false,
		"Generate the bootstrap.json but don't exec envoy")

	c.flags.BoolVar(&c.disableCentralConfig, "no-central-config", false,
		"By default the proxy's bootstrap configuration can be customized "+
			"centrally. This requires that the command run on the same agent as the "+
			"proxy will and that the agent is reachable when the command is run. In "+
			"cases where either assumption is violated this flag will prevent the "+
			"command attempting to resolve config from the local agent.")

	c.flags.StringVar(&c.grpcAddr, "grpc-addr", os.Getenv(api.GRPCAddrEnvName),
		"Set the agent's gRPC address and port (in http(s)://host:port format). "+
			"Alternatively, you can specify CONSUL_GRPC_ADDR in ENV.")

	// Deprecated, no longer needed, keeping it around to not break back compat
	c.flags.StringVar(&c.envoyVersion, "envoy-version", defaultEnvoyVersion,
		"This is a legacy flag that is currently not used but was formerly used to set the "+
			"version for the envoy binary that gets invoked by Consul. This is no longer "+
			"necessary as Consul will invoke the binary at a path set by -envoy-binary "+
			"or whichever envoy binary it finds in $PATH")

	c.flags.BoolVar(&c.register, "register", false,
		"Register a new gateway service before configuring and starting Envoy")

	c.flags.Var(&c.lanAddress, "address",
		"LAN address to advertise in the gateway service registration")

	c.flags.Var(&c.wanAddress, "wan-address",
		"WAN address to advertise in the gateway service registration. For ingress gateways, "+
			"only an IP address (without a port) is required.")

	c.flags.Var(&c.bindAddresses, "bind-address", "Bind "+
		"address to use instead of the default binding rules given as `<name>=<ip>:<port>` "+
		"pairs. This flag may be specified multiple times to add multiple bind addresses.")

	c.flags.StringVar(&c.gatewaySvcName, "service", "",
		"Service name to use for the registration")

	c.flags.BoolVar(&c.exposeServers, "expose-servers", false,
		"Expose the servers for WAN federation via this mesh gateway")

	c.flags.StringVar(&c.deregAfterCritical, "deregister-after-critical", "6h",
		"The amount of time the gateway services health check can be failing before being deregistered")

	c.flags.BoolVar(&c.omitDeprecatedTags, "omit-deprecated-tags", false,
		"In Consul 1.9.0 the format of metric tags for Envoy clusters was updated from consul.[service|dc|...] to "+
			"consul.destination.[service|dc|...]. The old tags were preserved for backward compatibility,"+
			"but can be disabled with this flag.")

	c.flags.StringVar(&c.prometheusBackendPort, "prometheus-backend-port", "",
		"Sets the backend port for the 'prometheus_backend' cluster that envoy_prometheus_bind_addr will point to. "+
			"Without this flag, envoy_prometheus_bind_addr would point to the 'self_admin' cluster where Envoy metrics are exposed. "+
			"The metrics merging feature in consul-k8s uses this to point to the merged metrics endpoint combining Envoy and service metrics. "+
			"Only applicable when envoy_prometheus_bind_addr is set in proxy config.")

	c.flags.StringVar(&c.prometheusScrapePath, "prometheus-scrape-path", "/metrics",
		"Sets the path where Envoy will expose metrics on envoy_prometheus_bind_addr listener. "+
			"For example, if envoy_prometheus_bind_addr is 0.0.0.0:20200, and this flag is "+
			"set to /scrape-metrics, prometheus metrics would be scrapeable at "+
			"0.0.0.0:20200/scrape-metrics. "+
			"Only applicable when envoy_prometheus_bind_addr is set in proxy config.")

	c.http = &flags.HTTPFlags{}
	flags.Merge(c.flags, c.http.ClientFlags())
	flags.Merge(c.flags, c.http.MultiTenancyFlags())
	c.help = flags.Usage(help, c.flags)
}

// canBindInternal is here mainly so we can unit test this with a constant net.Addr list
func canBindInternal(addr string, ifAddrs []net.Addr) bool {
	if addr == "" {
		return false
	}

	ip := net.ParseIP(addr)
	if ip == nil {
		return false
	}

	ipStr := ip.String()

	for _, addr := range ifAddrs {
		switch v := addr.(type) {
		case *net.IPNet:
			if v.IP.String() == ipStr {
				return true
			}
		default:
			if addr.String() == ipStr {
				return true
			}
		}
	}

	return false
}

func canBind(addr api.ServiceAddress) bool {
	ifAddrs, err := net.InterfaceAddrs()
	if err != nil {
		return false
	}

	return canBindInternal(addr.Address, ifAddrs)
}

func (c *cmd) Run(args []string) int {
	if err := c.flags.Parse(args); err != nil {
		return 1
	}

	// Setup Consul client
	var err error
	c.client, err = c.http.APIClient()
	if err != nil {
		c.UI.Error(fmt.Sprintf("Error connecting to Consul agent: %s", err))
		return 1
	}
	// TODO: refactor
	return c.run(c.flags.Args())
}

func (c *cmd) run(args []string) int {

	if c.nodeName != "" && c.proxyID == "" {
		c.UI.Error("'-node-name' requires '-proxy-id'")
		return 1
	}

	// Fixup for deprecated mesh-gateway flag
	if c.meshGateway && c.gateway != "" {
		c.UI.Error("The mesh-gateway flag is deprecated and cannot be used alongside the gateway flag")
		return 1
	}

	if c.meshGateway {
		c.gateway = meshGatewayVal
	}

	if c.exposeServers {
		if c.gateway != meshGatewayVal {
			c.UI.Error("'-expose-servers' can only be used for mesh gateways")
			return 1
		}
		if !c.register {
			c.UI.Error("'-expose-servers' requires '-register'")
			return 1
		}
	}

	// Gateway kind is set so that it is available even if not auto-registering the gateway
	if c.gateway != "" {
		kind, ok := supportedGateways[c.gateway]
		if !ok {
			c.UI.Error("Gateway must be one of: terminating, mesh, or ingress")
			return 1
		}
		c.gatewayKind = kind

		if c.gatewaySvcName == "" {
			c.gatewaySvcName = string(c.gatewayKind)
		}
	}

	if c.proxyID == "" {
		switch {
		case c.sidecarFor != "":
			proxyID, err := proxyCmd.LookupProxyIDForSidecar(c.client, c.sidecarFor)
			if err != nil {
				c.UI.Error(err.Error())
				return 1
			}
			c.proxyID = proxyID

		case c.gateway != "" && !c.register:
			gatewaySvc, err := proxyCmd.LookupGatewayProxy(c.client, c.gatewayKind)
			if err != nil {
				c.UI.Error(err.Error())
				return 1
			}
			c.proxyID = gatewaySvc.ID
			c.gatewaySvcName = gatewaySvc.Service

		case c.gateway != "" && c.register:
			c.proxyID = c.gatewaySvcName

		}
	}
	if c.proxyID == "" {
		c.UI.Error("No proxy ID specified. One of -proxy-id, -sidecar-for, or -gateway is " +
			"required")
		return 1
	}

	if c.register {
		if c.nodeName != "" {
			c.UI.Error("'-register' cannot be used with '-node-name'")
			return 1
		}
		if c.gateway == "" {
			c.UI.Error("Auto-Registration can only be used for gateways")
			return 1
		}

		taggedAddrs := make(map[string]api.ServiceAddress)
		lanAddr := c.lanAddress.Value()
		if lanAddr.Address != "" {
			taggedAddrs[structs.TaggedAddressLAN] = lanAddr
		}

		wanAddr := c.wanAddress.Value()
		if wanAddr.Address != "" {
			taggedAddrs[structs.TaggedAddressWAN] = wanAddr
		}

		tcpCheckAddr := lanAddr.Address
		if tcpCheckAddr == "" {
			// fallback to localhost as the gateway has to reside in the same network namespace
			// as the agent
			tcpCheckAddr = "127.0.0.1"
		}

		var proxyConf *api.AgentServiceConnectProxyConfig
		if len(c.bindAddresses.value) > 0 {
			// override all default binding rules and just bind to the user-supplied addresses
			proxyConf = &api.AgentServiceConnectProxyConfig{
				Config: map[string]interface{}{
					"envoy_gateway_no_default_bind": true,
					"envoy_gateway_bind_addresses":  c.bindAddresses.value,
				},
			}
		} else if canBind(lanAddr) && canBind(wanAddr) {
			// when both addresses are bindable then we bind to the tagged addresses
			// for creating the envoy listeners
			proxyConf = &api.AgentServiceConnectProxyConfig{
				Config: map[string]interface{}{
					"envoy_gateway_no_default_bind":       true,
					"envoy_gateway_bind_tagged_addresses": true,
				},
			}
		} else if !canBind(lanAddr) && lanAddr.Address != "" {
			c.UI.Error(fmt.Sprintf("The LAN address %q will not be bindable. Either set a bindable address or override the bind addresses with -bind-address", lanAddr.Address))
			return 1
		}

		var meta map[string]string
		if c.exposeServers {
			meta = map[string]string{structs.MetaWANFederationKey: "1"}
		}

		svc := api.AgentServiceRegistration{
			Kind:            c.gatewayKind,
			Name:            c.gatewaySvcName,
			ID:              c.proxyID,
			Address:         lanAddr.Address,
			Port:            lanAddr.Port,
			Meta:            meta,
			TaggedAddresses: taggedAddrs,
			Proxy:           proxyConf,
			Check: &api.AgentServiceCheck{
				Name:                           fmt.Sprintf("%s listening", c.gatewayKind),
				TCP:                            ipaddr.FormatAddressPort(tcpCheckAddr, lanAddr.Port),
				Interval:                       "10s",
				DeregisterCriticalServiceAfter: c.deregAfterCritical,
			},
		}

		if err := c.client.Agent().ServiceRegister(&svc); err != nil {
			c.UI.Error(fmt.Sprintf("Error registering service %q: %s", svc.Name, err))
			return 1
		}

		if !c.bootstrap {
			// We need stdout to be reserved exclusively for the JSON blob, so
			// we omit logging this to Info which also writes to stdout.
			c.UI.Info(fmt.Sprintf("Registered service: %s", svc.Name))
		}
	}

	// Generate config
	bootstrapJson, err := c.generateConfig()
	if err != nil {
		c.UI.Error(err.Error())
		return 1
	}

	if c.bootstrap {
		// Just output it and we are done
		c.UI.Output(string(bootstrapJson))
		return 0
	}

	// Find Envoy binary
	binary, err := c.findBinary()
	if err != nil {
		c.UI.Error("Couldn't find envoy binary: " + err.Error())
		return 1
	}

	err = execEnvoy(binary, nil, args, bootstrapJson)
	if err == errUnsupportedOS {
		c.UI.Error("Directly running Envoy is only supported on linux and macOS " +
			"since envoy itself doesn't build on other platforms currently.")
		c.UI.Error("Use the -bootstrap option to generate the JSON to use when running envoy " +
			"on a supported OS or via a container or VM.")
		return 1
	} else if err != nil {
		c.UI.Error(err.Error())
		return 1
	}

	return 0
}

var errUnsupportedOS = errors.New("envoy: not implemented on this operating system")

func (c *cmd) findBinary() (string, error) {
	if c.envoyBin != "" {
		return c.envoyBin, nil
	}
	return exec.LookPath("envoy")
}

func (c *cmd) templateArgs() (*BootstrapTplArgs, error) {
	httpCfg := api.DefaultConfig()
	c.http.MergeOntoConfig(httpCfg)

	// api.NewClient normalizes some values (Token, Scheme) on the Config.
	if _, err := api.NewClient(httpCfg); err != nil {
		return nil, err
	}

	xdsAddr, err := c.xdsAddress(httpCfg)
	if err != nil {
		return nil, err
	}

	adminAddr, adminPort, err := net.SplitHostPort(c.adminBind)
	if err != nil {
		return nil, fmt.Errorf("Invalid Consul HTTP address: %s", err)
	}

	// Envoy requires IP addresses to bind too when using static so resolve DNS or
	// localhost here.
	adminBindIP, err := net.ResolveIPAddr("ip", adminAddr)
	if err != nil {
		return nil, fmt.Errorf("Failed to resolve admin bind address: %s", err)
	}

	// Ideally the cluster should be the service name. We may or may not have that
	// yet depending on the arguments used so make a best effort here. In the
	// common case, even if the command was invoked with proxy-id and we don't
	// know service name yet, we will after we resolve the proxy's config in a bit
	// and will update this then.
	cluster := c.proxyID
	proxySourceService := ""
	if c.sidecarFor != "" {
		cluster = c.sidecarFor
		proxySourceService = c.sidecarFor
	} else if c.gateway != "" && c.gatewaySvcName != "" {
		cluster = c.gatewaySvcName
		proxySourceService = c.gatewaySvcName
	}

	adminAccessLogPath := c.adminAccessLogPath
	if adminAccessLogPath == "" {
		adminAccessLogPath = DefaultAdminAccessLogPath
	}

	var caPEM string
	pems, err := tlsutil.LoadCAs(httpCfg.TLSConfig.CAFile, httpCfg.TLSConfig.CAPath)
	if err != nil {
		return nil, err
	}
	caPEM = strings.Replace(strings.Join(pems, ""), "\n", "\\n", -1)

	return &BootstrapTplArgs{
		GRPC:                  xdsAddr,
		ProxyCluster:          cluster,
		ProxyID:               c.proxyID,
		NodeName:              c.nodeName,
		ProxySourceService:    proxySourceService,
		AgentCAPEM:            caPEM,
		AdminAccessLogPath:    adminAccessLogPath,
		AdminBindAddress:      adminBindIP.String(),
		AdminBindPort:         adminPort,
		Token:                 httpCfg.Token,
		LocalAgentClusterName: xds.LocalAgentClusterName,
		Namespace:             httpCfg.Namespace,
		Partition:             httpCfg.Partition,
		Datacenter:            httpCfg.Datacenter,
		PrometheusBackendPort: c.prometheusBackendPort,
		PrometheusScrapePath:  c.prometheusScrapePath,
	}, nil
}

func (c *cmd) generateConfig() ([]byte, error) {
	args, err := c.templateArgs()
	if err != nil {
		return nil, err
	}

	var bsCfg BootstrapConfig

	// Fetch any customization from the registration
	var svcProxyConfig *api.AgentServiceConnectProxyConfig
	var serviceName, ns, partition, datacenter string
	if c.nodeName == "" {
		svc, _, err := c.client.Agent().Service(c.proxyID, nil)
		if err != nil {
			return nil, fmt.Errorf("failed fetch proxy config from local agent: %s", err)
		}
		svcProxyConfig = svc.Proxy
		serviceName = svc.Service
		ns = svc.Namespace
		partition = svc.Partition
		datacenter = svc.Datacenter
	} else {
		filter := fmt.Sprintf("ID == %q", c.proxyID)
		svcList, _, err := c.client.Catalog().NodeServiceList(c.nodeName, &api.QueryOptions{Filter: filter})
		if err != nil {
			return nil, fmt.Errorf("failed to fetch proxy config from catalog for node %q: %w", c.nodeName, err)
		}
		if len(svcList.Services) != 1 {
			return nil, fmt.Errorf("expected to find only one proxy service with ID: %q", c.proxyID)
		}
		svcProxyConfig = svcList.Services[0].Proxy
		serviceName = svcList.Services[0].Service
		ns = svcList.Services[0].Namespace
		partition = svcList.Services[0].Partition
		datacenter = svcList.Node.Datacenter
		c.gatewayKind = svcList.Services[0].Kind
	}
	if svcProxyConfig == nil {
		return nil, errors.New("service is not a Connect proxy or gateway")
	}

	if svcProxyConfig.DestinationServiceName != "" {
		// Override cluster now we know the actual service name
		args.ProxyCluster = svcProxyConfig.DestinationServiceName
		args.ProxySourceService = svcProxyConfig.DestinationServiceName
	} else {
		// Set the source service name from the proxy's own registration
		args.ProxySourceService = serviceName
	}

	// In most cases where namespaces and partitions are enabled they will already be set
	// correctly because the http client that fetched this will provide them explicitly.
	// However, if these arguments were not provided, they will be empty even
	// though Namespaces and Partitions are actually being used.
	// Overriding them ensures that we always set the Namespace and Partition args
	// if the cluster is using them. This prevents us from defaulting to the "default"
	// when a non-default partition or namespace was inferred from the ACL token.
	if ns != "" {
		args.Namespace = ns
	}
	if partition != "" {
		args.Partition = partition
	}

	if datacenter != "" {
		// The agent will definitely have the definitive answer here.
		args.Datacenter = datacenter
	}

	// Setup ready listener for ingress gateway to pass healthcheck
	if c.gatewayKind == api.ServiceKindIngressGateway {
		lanAddr := c.lanAddress.String()
		// Deal with possibility of address not being specified and defaulting to
		// ":443"
		if strings.HasPrefix(lanAddr, ":") {
			lanAddr = "127.0.0.1" + lanAddr
		}
		bsCfg.ReadyBindAddr = lanAddr
	}

	if !c.disableCentralConfig {
		// Parse the bootstrap config
		if err := mapstructure.WeakDecode(svcProxyConfig.Config, &bsCfg); err != nil {
			return nil, fmt.Errorf("failed parsing Proxy.Config: %s", err)
		}
	}

	return bsCfg.GenerateJSON(args, c.omitDeprecatedTags)
}

// TODO: make method a function
func (c *cmd) xdsAddress(httpCfg *api.Config) (GRPC, error) {
	g := GRPC{}

	addr := c.grpcAddr
	if addr == "" {
		port, err := c.lookupXDSPort()
		if err != nil {
			c.UI.Error(fmt.Sprintf("Error connecting to Consul agent: %s", err))
		}
		if port <= 0 {
			// This is the dev mode default and recommended production setting if
			// enabled.
			port = 8502
		}
		addr = fmt.Sprintf("localhost:%v", port)
	}

	// TODO: parse addr as a url instead of strings.HasPrefix/TrimPrefix

	// Decide on TLS if the scheme is provided and indicates it, if the HTTP env
	// suggests TLS is supported explicitly (CONSUL_HTTP_SSL) or implicitly
	// (CONSUL_HTTP_ADDR) is https://
	switch {
	case strings.HasPrefix(strings.ToLower(addr), "https://"):
		g.AgentTLS = true
	case httpCfg.Scheme == "https":
		g.AgentTLS = true
	}

	// We want to allow grpcAddr set as host:port with no scheme but if the host
	// is an IP this will fail to parse as a URL with "parse 127.0.0.1:8500: first
	// path segment in URL cannot contain colon". On the other hand we also
	// support both http(s)://host:port and unix:///path/to/file.
	if grpcAddr := strings.TrimPrefix(addr, "unix://"); grpcAddr != addr {
		// Path to unix socket
		g.AgentSocket = grpcAddr
	} else {
		// Parse as host:port with option http prefix
		grpcAddr = strings.TrimPrefix(addr, "http://")
		grpcAddr = strings.TrimPrefix(grpcAddr, "https://")

		var err error
		var host string
		host, g.AgentPort, err = net.SplitHostPort(grpcAddr)
		if err != nil {
			return g, fmt.Errorf("Invalid Consul HTTP address: %s", err)
		}

		// We use STATIC for agent which means we need to resolve DNS names like
		// `localhost` ourselves. We could use STRICT_DNS or LOGICAL_DNS with envoy
		// but Envoy resolves `localhost` differently to go on macOS at least which
		// causes paper cuts like default dev agent (which binds specifically to
		// 127.0.0.1) isn't reachable since Envoy resolves localhost to `[::]` and
		// can't connect.
		agentIP, err := net.ResolveIPAddr("ip", host)
		if err != nil {
			return g, fmt.Errorf("Failed to resolve agent address: %s", err)
		}
		g.AgentAddress = agentIP.String()
	}
	return g, nil
}

func (c *cmd) lookupXDSPort() (int, error) {
	self, err := c.client.Agent().Self()
	if err != nil {
		return 0, err
	}

	type response struct {
		XDS struct {
			Port int
		}
	}

	var resp response
	if err := mapstructure.Decode(self, &resp); err == nil && resp.XDS.Port != 0 {
		return resp.XDS.Port, nil
	}

	// Fallback to old API for the case where a new consul CLI is being used with
	// an older API version.
	cfg, ok := self["DebugConfig"]
	if !ok {
		return 0, fmt.Errorf("unexpected agent response: no debug config")
	}
	port, ok := cfg["GRPCPort"]
	if !ok {
		return 0, fmt.Errorf("agent does not have grpc port enabled")
	}
	portN, ok := port.(float64)
	if !ok {
		return 0, fmt.Errorf("invalid grpc port in agent response")
	}

	return int(portN), nil
}

func (c *cmd) Synopsis() string {
	return synopsis
}

func (c *cmd) Help() string {
	return c.help
}

const (
	synopsis = "Runs or Configures Envoy as a Connect proxy"
	help     = `
Usage: consul connect envoy [options] [-- pass-through options]

  Generates the bootstrap configuration needed to start an Envoy proxy instance
  for use as a Connect sidecar for a particular service instance. By default it
  will generate the config and then exec Envoy directly until it exits normally.

  It will search $PATH for the envoy binary but this can be overridden with
  -envoy-binary.

  It can instead only generate the bootstrap.json based on the current ENV and
  arguments using -bootstrap.

  The proxy requires service:write permissions for the service it represents.
  The token may be passed via the CLI or the CONSUL_HTTP_TOKEN environment
  variable.

  The example below shows how to start a local proxy as a sidecar to a "web"
  service instance. It assumes that the proxy was already registered with it's
  Config for example via a sidecar_service block.

    $ consul connect envoy -sidecar-for web

  Additional arguments may be passed directly to Envoy by specifying a double
  dash followed by a list of options.

    $ consul connect envoy -sidecar-for web -- --log-level debug
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